1. Field of the Invention
This invention relates to conductive resin compositions and also to information records using such compositions. The conductive resin compositions are particularly suitable for use as information records such as video discs or digital audio discs but may be effectively utilized in other fields such as packaging. Only for convenience' sake, the compositions may be chiefly described with reference to information records.
2. Description of the Prior Art
Information playback systems of an electrostatic capacitance type are known in the art, in which signal information is recorded as geometric variations or pits in a spiral plane or groove, or in concentric planes or grooves of an information record. The record is reproduced by scanning it with a pickup stylus, such as of diamond, having an electrode thereon along the record track, whereupon capacitive variations are established between the pickup stylus and the record according to the geometric variations in the plane or groove and are then converted back to, for example, video information suitable for display by a television receiver.
Typical of the known information records used in the above type of electrostatic capacitance playback system is a record which has geometric variations press-molded according to intended signal information, on which an electrode formed on the surface of the record. The electrode can form an electrostatic capacitance in association with the stylus electrode. In practice, the record substrate is covered with a thin metallic film electrode of several hundreds angstrom in thickness and a several hundred angstrom thick dielectric layer overlying the metallic film electrode. The dielectric layer prevents short circuiting of the electrodes and increases the dielectric constant between the electrodes. The record of this type needs a number of fabricating steps including a step of press molding a record substrate, steps of depositing the thin metallic film and the dielectric film, and the like. Thus, the fabrication is complicated and troublesome with the need of a relatively large-scale manufacturing apparatus. Thus, fabrication costs become very high.
Another type of known information record of the electrostatic capacitance type is a record which is obtained from conductive plastic compositions. The conductive plastic compositions are mixtures of thermoplastic resins, conductive materials, and other additives. Thermoplastic resins used for these purposes are, for example, vinyl chloride resins, vinyl chloride-vinyl acetate copolymers, vinyl chloride-ethylene copolymers, vinyl chloride-propylene copolymers, vinyl chloride-alkyl acrylate copolymers, vinyl chloride-acrylonitrile-styrene copolymers, graft copolymers of vinyl chloride to ethylene-vinyl acetate copolymers, vinyl chloride-alpha-olefin copolymers, vinyl chloride-vinyl alcohol copolymers, and the like. The additives may include: metallic soaps such as calcium stearate; organic tin stabilizers; lubricants such as higher alcohols, higher fatty acids, fatty acid esters, esters of polysaccharides, and the like; lubricating oils such as dimethylpolysiloxane; and the like. A typical conductive material is carbon black, which is used in amounts necessary for the electrostatic capacitance playback systems. Carbon blacks useful for these purposes may be commercially available, for example, under designations of Ketjen Black from Akzo Cheme Co., Ltd., CSX-150A3from Cabot Co., Ltd., and XE-2 from Phillips Petroleum Chemicals Co., Ltd. These ingredients are blended, for example, in a high speed mixer such as Henschel mixer in order that carbon black is appropriately dispersed in the resin. The resulting mixture is molten in a highly kneadable extruder and pelletized, followed by shaping the pellets in a disc pressing machine to obtain video or digital audio discs. This disc is conductive and is able to establish an electrostatic capacitance in association with the electrode of a playback stylus. The record obtained from conductive resin compositions is advantageous in that no steps of depositing a thin metallic film and a thin dielectric layer are needed. Thus, such disc can be fabricated simply at low costs.
However, in order to obtain conductive resin compositions, based on vinyl chloride resins and carbon black, which have characteristic properties necessary for the electrostatic capacitance playback systems, it is essential that carbon black be used in amounts of at least 5 to 30 wt%, preferably over about 10 wt%. When such a large amount of carbon black is used, the resulting composition ordinarily exhibits poor melt-flowability and thus poor moldability. In addition, carbon black has the tendency of impeding the thermal stability of vinyl chloride resins.
Thus, there is a demand for conductive resin compositions which are very useful as information records for use in electrostatic capacitance playback systems and which may be also used in antistatic fields such as a packaging field.